Thin film electroluminescent element

ABSTRACT

In a thin film electroluminescent element comprising a phosphor thin film, a dielectric thin film disposed on at least one of the surfaces of the phosphor thin film and electrodes for applying a voltage across the thin films, the aforementioned dielectric thin film is formed of a dielectric material expressed by a general formula of AB 2  O 6  (where A represents a divalent metal element and B represents a pentavalent metal element). By employing the dielectric material, the voltage for driving a thin film electroluminescent element can be lowered without decreasing the brightness of the element. Further, by using a composite dielectric thin film in which the dielectric thin film expressed by the aforementioned general formula AB 2  O 6  is laminated with a dielectric thin film which is not susceptible to dielectric breakdown of self-healing type, the composite dielectric thin film is made susceptible to the dielectric breakdown of self-healing type. Additionally, the value of product of the dielectric breakdown field intensity and dielectric constant is increased to obtain a thin film electroluminescent element having excellent characteristics.

This invention relates to a thin film luminescent element which producesluminescence under application of an electric field.

BACKGROUND OF THE INVENTION

In a thin film EL (electroluminescent) element which producesluminescence in response to the application of an electric field,increased brightness is attempted to be attained by sandwiching aphosphor thin film, onto which one or both surfaces thereof is depositeda dielectric thin film, between two electrode layers. The element forwhich the dielectric thin film is deposited on one surface of thephosphor thin film is characterized by a simplified structure and a lowdriving voltage. The element for which both surfaces of the phosphorthin film layer have dielectric thin films deposited thereon,respectively, is advantageous in that it is less easy for dielectricbreakdown to occur and that brightness is significantly increased. It isknown to use ZnS, ZnSe, ZnF₂ or the like added with an activator for thephosphor material. In particular, in the case of an element employingphosphor which is composed of ZnS as the host material and contains Mnas the activator for light emission, brightness in the range of 3500 to5000 cd/m² at maximum is attained. As the typical dielectric material,Y₂ O₃, SiO, Si₃ N₄, Al₂ O₃, Ta₂ O₅ and the like may be used. The layerof ZnS has a thickness in the range of 500 to 700 nm and a dielectricconstant of about 9. The thickness of the dielectric film is in therange of 400 to 800 nm and its dielectric constant is in the range of 4to 25.

When the element is driven by using an AC voltage, the voltage appliedacross the element is divided between the layer of ZnS and thedielectric thin film, wherein about 40% to 60% of the voltage appliedacross the electrodes is found in the layer of ZnS. The voltage requiredfor producing brightness thus appears to be higher. In the case of theelement having both surfaces provided with dielectric thin films,brightness is produced by applying a voltage of 200 V or greater at afrequency on the order of KHz at the present state of art. Such a highvoltage imposes a great load on the driving circuit, requiring aspecial, expensive, integrated circuit (IC) capable of withstanding thehigh voltage.

In this connection, it is proposed to use as the dielectric thin film athin film which contains TbTiO₃, Pb(Ti_(1-x) Zr_(x))O₃ or the like asits main component and exhibits a high dielectric constant, for loweringthe driving voltage. Although this type thin film has a dielectricconstant (hereinafter represented by ε.sub.γ) as high as 100 or more,electric field intensity at which the dielectric breakdown occurs(hereinafter represented by E_(b)) is as low as 0.5 MV/cm, which meansthat the film thickness be significantly increased when compared withthat of the heretofore used dielectric material. In the case of anelement designed for high brightness, it is required that the thicknessof the ZnS-layer be on the order of 0.6 μm. Further, from the standpoint of reliability of the element, the aforementioned dielectric thinfilm has to be formed in thickness not smaller than 1.5 μm. Whentemperature of the substrate is high, increase in film thickness resultsin the growth of particles within the film. As the consequence, a filmbecomes turid and white, decreasing light transmission. In an EL elementin which such white-turbid film is employed and which is arranged in anX-Y matrix configuration, even a non-selected pixel will scatter lightemitted by other pixels, causing the troublesome problem of cross-talk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides sectional and top views illustrating a self-healing typedielectric breakdown in a dielectric layer, and FIG. 2 providessectitonal and top views illustrating a dielectric breakdown in adielectric layer which is not of the self-healing nature.

FIG. 3 is a sectional view of a thin film electroluminescent elementshown for the purpose of comparison with the element according to theinvention, and

FIG. 4 is a sectional view showing a thin film electroluminescentelement according to an exemplary embodiment of the present invention.

FIGS. 5 and 6 are sectional views showing, respectively, other exemplaryembodiments of the thin film electroluminescent element according tothis invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With the present invention, it is intended to solve the problemsdescribed hereinbefore. It is proposed according to the invention to usea dielectric layer which has a composition generally expressed by AB₂ O₆where A represents a divalent metal element, B represents a pentavalentmetal element and O represents oxygen, which exhibits high ε.sub.γ andE_(b) values, to thereby allow the driving voltage to be lowered withoutdecreasing the brightness when compared with hitherto known thin film ELelements.

In an AC-driven thin film EL element, the voltage applied across thedielectric layer is represented by the product t_(i) ·E_(i), where t_(i)represents the film thickness of the dielectric thin film and E_(i)represents the electric field intensity applied to the dielectric thinfilm. The voltage applied across the phosphor thin film becomes moreeffective as the value of t_(i) ·E_(i) is decreased. It is safe to saythat t_(i) is in inverse proportion to E_(b) of the dielectric thin filmin order that the element can operate stably without undergoingdielectric breakdown. Among E_(i), the electric field intensity E_(Z) inthe phosphor thin film, the dielectric constant ε_(Z) of the phosphorthin film and ε.sub.γ of the dielectric thin film, a relationship ofE_(i) =E_(z) ·ε_(z) /ε.sub.γ applies. E_(i) is in inverse proportion toε.sub.γ, providing E_(Z) and ε_(Z) are constant. Accordingly, it can besaid that t_(i) ·E_(i) is approximately in inverse proportion to theproduct of E_(b) and ε_(r). The dielectric thin film is moreadvantageous with E_(b) ·ε.sub.γ of higher value.

The dielectric thin film defined by the general formula of AB₂ O₆ andused according to the teaching of the present invention, exhibits E_(b)·ε.sub.γ of a greater value than that of the heretofore used materialand is preferable as the dielectric thin film for the EL element. Inconnection with the above formula, A represents a divalent metal elementsuch as Pb, Sn, Zn, Cd, Ba, Sr, Ca and Mg, and B represents Ta or Nb. Amass of a compound of these elements exhibits ε.sub.γ of a great value.By way of example, it is reported that the ε.sub.γ of PbNb₂ O₆ is 300,the ε.sub.γ of PbTa₂ O₆ is 300 and the ε.sub.γ of (Pb₀.55 Sr₀.45)Nb₂ O₆is 1600. In the case of a thin film, it is difficult to realize anε.sub.γ of the same value as a mass of the same material. Howeverε.sub.γ of a value not less than 40 can be easily realized in a thinfilm fabricated by a sputtering process. In addition, E_(b) of the thinfilm is as high as 2×10⁶ V/cm or more. The value of E_(b) ·ε.sub.γ ofsuch thin film is not less than 80×10⁶ V/cm. It will be seen that thethin film formed from the compound mentioned above is excellent whencompared with the material used heretofore such as, for example, Y₂ O₃,Al₂ O₃ and Si₃ N₄ whose values of E_(b) ·ε.sub.γ are about 50×10⁶ V/cm,30×10⁶ V/cm and 70×10⁶ V/cm, respectively. In the compound expressed bythe general formula of AB₂ O₆, Nb and Ta, which are most stable inpentavalence, are preferrable as the element represented by B. Among thedivalent elemnts represented by A, Sr, Ba and Pb are very preferable.Above all, PbTa₂ O₆, and PbNb₂ O₆ where the element represented by A isPb, which have the values of E_(b) ·ε.sub.γ of 150×10⁶ V/cm and 120×10⁶V/cm, respectively, provide very excellent thin film materials for theEL element. The thin film is formed by an RF sputtering method with aceramic target. As the temperature of the substrate on which the thinfilm is to be formed is increased, the value of ε.sub.γ of the thin filmas formed becomes correspondingly greater. The dielectric breakdownfield intensity E_(b) assumes a substantially constant value when thetemperature of the substate is lower than about 400° C. and is graduallydecreased when the substrate temperature is raised to a highertemperature. The value of E_(b) ·ε.sub.γ becomes greatest when thetemperature of the substrate is approximately at 400° C. In thetemperature range mentioned above, no adverse influence will be exertedon the phosphor thin film. Besides, glass may be used as the materialfor the substrate without giving rise to a problem such as thermaldeformation of the substrate. Moreover, no white turbidity will beproduced due to the growth of particles.

Unless the temperature of the substrate is sufficiently high, the thinfilm will be found to be amorphous when investigated by means of X-raydiffraction. Through chemical analysis and phosphor X-ray analysis, ithas been ascertained that the thin film has a composition substantiallycoinciding with the general formula of AB₂ O₆.

In general, various defects are produced in the thin film by pinholes,dusts and the like. When a voltage is applied to the dielectric thinfilm, dielectric breakdown is likely to take place at the defectivelocations at a lower voltage rather than the non-defective locations.

The dielectric breakdown may generally be classified into two types. Oneis the dielectric breakdown of self-healing type. More specifically,referring to FIG. 1, an upper electrode 15 overlying a location 16 wherethe dielectric breakdown has occurred is eliminated from an area ofseveral tens of μm under discharging energy, wherein the upper electrode15 is disconnected from a lower electrode 12. The dielectric breakdownoccurring in the dielectric thin film of the composition expressed bythe general formula AB₂ O₆, where A represents a divalent metal elementand B represents a pentavalent metal element, is of this type. Thenumeral 11 denotes a substrate, and 13 denotes a dielectric thin filmwhich is the dielectric breakdown of the self-healing type. As is shownin FIG. 2, the upper electrode 25 is eliminated only to such a smalldegree that the upper electrode 25 is electrically short-circuited tothe lower electrode 22 on substrate 21 through a hole 26 formed by thedielectric breakdown in dielectric thin film 23 which is not susceptibleto dielectric breakdown of the self-healing type. When the voltagecontinues to be applied in this state, the dielectric breakdown mayspread over the whole dielectric film. A dielectric thin film containingperovskite type titanate as a main component is of this type.

As the thickness of the upper electrode is decreased, the dielectricbreakdown is less likely to occur. However, if the thickness isdecreased excessively, resistance of the electrode is increased, to adisadvantage. Accordingly, the electrode should have a thickness ofseveral tens of nm's at minimum. Electrode material such as Au, Zn, Aland others is most likely to undergo the dielectric breakdown of theself-healing type. However, there exist some dielectric thin films inwhich no dielectric breakdown of the self-healing type takes place evenwhen the electrode is of Au, Zn, Al or the like having a thickness ofseveral tens of nm's. This dielectric breakdown is ascribable to theinherent nature of the material. Although the reason can not beexplained, it appears that the aspect of the arc-discharge which isproduced upon dielectric breakdown, which is effective to eliminate thematerial of the upper, differs between a film in which dielectricbreakdown of the self-healing type will occur and a film whosedielectric breakdown is not of the self-healing nature.

In case a dielectric thin film whose dielectric breakdown is of theself-healing type is used as the dielectric thin film formed on thephosphor layer of an AC-driven thin film EL element, the dielectricbreakdown occurring at the defective portion is of the self-healingtype. The material of the upper electrode is eliminated over an area ofseveral tens of um's. Since an eliminated pinhole can not be visiblyrecognized, the dielectric breakdown of the self-healing type presentsno practical problem. Since the dielectric thin film of the compositionexpressed by the general formula of AB₂ O₆ (where A represents adivalent metal element and B represents a pentavalent metal element) issusceptible to the dielectric breakdown of this type, it is preferred asthe dielectric thin film for the AC-driven thin film EL element inrespect to dielectric breakdown. On the other hand, when the dielectricfilm whose dielectric breakdown is not of the self-healing type isformed on the phosphor layer of the AC-driven thin film EL-element, adielectric breakdown occurring at the defective portion is of the secondmentioned type. The dielectric breakdown is likely to spread over thewhole pixel, producing a visible deficiency. In the case of an X-Ymatrix array, a line defect will result. Although the thin film ofperovskite type titanate can be easily fabricated with a large value ofε.sub.γ and exhibit E_(b) of a large value at the locations where nodefects due to pinholes and dusts are present, this film is notsusceptible to dielectric breakdown of the self-healing type. Inparticular, in the case of a thin film of strontium titanate or bariumtitanate having ε.sub. γ of a great value, the dielectric breakdown ofthe self-healing type occurs with difficulty, these thin films were notused for the AC-driven thin film EL element. However, when thedielectric thin film of the composition expressed by the general formulaof AB₂ O₆ mentioned above is formed on a thin film of the abovementioned type, the dielectric breakdown occurring due to pinholes anddusts is advantageously of a self-healing nature. In this way, by usinga composite dielectric film formed by superimposing a dielectric thinfilm having a larger value of E_(b) ·ε.sub.γ than the film expressed bythe general formula of AB₂ O₆, and not being susceptible to theself-healing type of dielectric breakdown, and the aforementioneddielectric thin film and that expressed by the general formula of AB₂ O₆being superimposed onto each other, a dielectric breakdown of thecomposite film takes place in the form of the self-healing breakdown,while an E_(b) ·ε.sub.γ of a larger value than that of theaforementioned dielectric thin film represented by the general formulaof AB₂ O₆ can be assured. It is desirable that the E_(b) ·ε.sub.γ of adielectric thin film which is not susceptible to self-healing typedielectric breakdown is not smaller than 80×10⁶.

Next, exemplary embodiments of the present invention will be describedby referring to the drawings.

For facilitating understanding, the description will be made inconjunction with a comparative example. FIG. 3 shows the comparativeexample, and FIG. 4 shows an exemplary embodiment of the presentinvention. As is apparent from the drawings, Y₂ O₃ -films 33 and 43,each of 40 nm in thickness, were formed by an electron beam evaporatingmethod on glass substrates 31 and 41 deposited with transparentelectrodes 32 and 42 of ITO (indium tin oxide), respectively.Subsequently, phosphor layers 34 and 44 of ZnS:Mn were formed throughsimultaneous evaporation of ZnS and Mn. Film thickness is 600 nm. Heattreatment was carried out at 580° C. in vacuum for one hour. Theelements were divided into five elements, one of which was used as aspecimen for comparison, and a Y₂ O₃ -film 35 of 400 nm thick wasformed, as is shown in FIG. 3. On the other hand, the element 2 wasformed with a Ta₂ O₅ -film 45 of 30 nm in thickness for the protectionof ZnS:Mn by an electron beam evaporating method, as is shown in FIG. 4,in accordance with an embodiment of the present invention. Subsequently,a film 46 of PbNb₂ O₆ was formed through magnetron RF sputtering byusing a ceramic of PbNb₂ O₆ as a target. The atmosphere for thesputtering contained O₂ and Ar at the ratio of 1:4 at a pressure of 0.6Pa. The temperature of the substrate was 420° C. and the film thicknesswas 700 nm. According to another embodiment of the present invention,the element 3 was formed with a film of PbTa₂ O₆ in a thickness of 700nm on the same conditions as in the case of the element 2, except that atarget of PbTa₂ O₆ was employed in place of PbNb₂ O₆.

In accordance with still another embodiment of the present invention,the element 4 was formed with a film of BaTa₂ O₆ in a thickness of 500nm on the same conditions as in the case of the element 2, except thatBaTa₂ O₆ was used in place of PbNb₂ O₆ as the target.

According to a further embodiment of the present invention, the element5 was formed with a film of SrTa₂ O₆ in a thickness of 450 nm on thesame conditions as is the case of the element 2, except that SrTa₂ O₆was used in place of PbNb₂ O₆ as the target.

The PbNb₂ O₆ -film, the PbTa₂ O₆ -film, the BaTa₂ O₆ -film and the SrTa₂O₆ -film fabricated under the aforementioned conditions havecharacteristic E_(b) 's of 2.2×10⁶ V/cm, 2.6×10⁶ V/cm, 5.1×10⁶ V/cm and5.6×10⁶ V/cm, respectively, and ε.sub.γ 's of 70, 48, 27 and 25,respectively.

As is shown in FIGS. 3 and 4, thin films of Al were deposited throughvaporization to form light reflecting electrodes 36 and 47.

Each of the EL elements fabricated in the manner described above wasdriven by applying a sine wave voltage of a frequency of 5 KHz acrossthe electrodes. The voltage at which brightness was substantiallysaturated in the stable state was 150 V in the case of the element 1,100 V in the case of the element 2, 110 V in the case of the element 3,125 V in the case of the element 4 and 125 V in the case of the element5. The saturated brightness was about 3000 cd/m² in all of the fiveelements.

Next, an embodiment of this invention according to which an AC-driventhin film EL element having a dielectric layer only on one surface of aphosphor layer and in which tungsten bronze type composite oxide film isemployed will be described by referring to FIG. 5. A ZnO-film 53 havinga thickness of 50 nm was formed by a sputtering method on a glasssubstrate 51 deposited with a transparent electrode 52 of ITO. The film53 of ZnO has a resistivity of 8×10⁻³ Ω·cm and serves as a secondelectrode layer for preventing diffusion of In and Sn into ZnS from thetransparent electrode 52 of ITO. Subsequently, ZnS and Mn weresimultaneously evaporated to form a phosphor layer 54 of ZnS:Mn inthickness of 450 nm. Heat treatment was conducted at 580° C. in vacuumfor an hour. Further, a film 55 of Y₂ O₃ having thickness of 20 nm wasformed by an electron beam evaporating method for protecting thephosphor layer 54 of ZnS:Mn. Subsequently, a PbNb₂ O₆ -film 56 wasformed by a magnestron RF sputtering method by using ceramic of PbNb₂ O₆as a target. Composition of the sputtering atmosphere was O₂ :Ar=1:1 (involume ratio), and the pressure thereof was 1.3 Pa. The temperature ofthe substrate was 320° C. and the film thickness was 500 nm. The film 56of PbNb₂ O₆ fabricated on the conditions mentioned above hascharacteristic E_(b) of 2.5×10⁶ V/cm and ε.sub.γ of 56. Finally, anAl-thin film 57 was formed through evaporation as a light reflectingelectrode.

The EL element manufactured in the manner described above was driven byapplying a sine wave voltage of 5 KHz between the electrodes. Brightnesswas substantially saturated at about 70 V. In the stable state,brightness was 1900 cd/m².

A further embodiment of this invention will be described with the aid ofFIG. 6.

As is shown in FIG. 6, a glass substrate 61 having a transparentelectrode 62 of ITO was deposited with a Y₂ O₃ -film 63 in a thicknessof 40 nm through electron beam evaporation. Subsequently, a phosphorlayer 64 of ZnS:Mn was formed in a thickness of 1.0 μm by simultaneouslyevaporating ZnS and Mn through vacuum vapor deposition. Heat treatmentwas conducted at 580° C. in vacuum for one hour. Thereafter, a Ta₂ O₅-film 65 was deposited in a thickness of 40 nm through electron beamevaporation for protecting the film of ZnS:Mn. The element was dividedinto two, one of which was deposited with a SrTiO₃ -film in a thicknessof 1.4 μm while the other was deposited with a BaTiO₃ -film in thicknessof 1.6 μm by a magnetron RF sputtering method. A mixed gas of O₂ and Arwas used as the sputtering gas at pressure of 8×10⁻¹ Pa. The temperatureof the substate at that time was 420° C. Additionally, a PbNb₂ O₆ -film67 was deposited in a thickness of 0.4 μm by a magnetron RF sputteringmethod. A mixed gas containing O₂ and Ar at the ratio of 1 to 1 was usedas the sputtering gas at a pressure of 0.6 Pa. A sintered body of PbNb₂O₆ was used as the target. The temperature of the substate was 380° C. Afilm 68 of Al was deposited in thickness of 70 nm to form the upperelectrode. A voltage was applied between the electrodes of the thin filmEL element thus manufactured and the applied voltage was progressivelyincreased. Before brightness was produced, dielectric breakdowns ofsmall degree occurred at defective portions to form holes in diameter ofabout 30 μm in the Al-film 68 by elimination of the film material. Thedielectric breakdowns were all of the self-healing type. The number ofthe breakdowns was 0.5/cm² in both elements. When the elements weredriven by applying an AC pulse voltage of 5 KHz. Both elements weredriven into the state in which brightness was substantially saturatedwhen zero-to-peak voltage of about 230 V was applied. The brightness wasabout 7000 cd/m².

As will be appreciated from the foregoing, the thin filmelectroluminescent element according to the invention can be operatedstably with a low driving voltage.

We claim:
 1. A thin film electroluminescent element comprising aphosphor thin film, a dielectric thin film disposed on at least onesurface of said phosphor thin film, and electrodes for applying avoltage across said films, wherein said dielectric thin film comprises adielectric material subject to dielectric breakdown of the self-healingtype having a composition expressed by the general formula of AB₂ O₆,where A is at least one divalent metal element selected from the groupconsisting of Pb, Sn, Mg, Ca, Sr, Ba, Zn and Cd, and B is at least onepentavalent metal element selected from the group consisting of Ta andNb, wherein the product E_(b) ·ε.sub.γ of the dielectric breakdownelectric field intensity E_(b) and dielectric constant ε.sub.γ for thedielectric thin film is greater than or equal to 80×10⁶ V/cm.
 2. A thinfilm electroluminescent element according to claim 1, wherein thedivalent metal element A is at least one selected from a groupconsisting of Pb, Sr and Ba.
 3. A thin film electroluminescent elementaccording to claim 1, wherein the divalent metal element A is Pb.
 4. Athin film electroluminescent element comprising a phosphor thin film, adielectric thin film disposed on at least one surface of said phosphorthin film, and electrodes for applying a voltage across said films,wherein said dielectric thin film comprises a dielectric material havinga composition expressed by the general formula of AB₂ O₆, wherein A isat least one divalent metal selected from a group consisting of Pb, Sn,Mg, Ca, Sr, Ba, Zn and Cd, and B is at least one pentavalent metalselected from the group consisting of Ta and Nb.
 5. A thin filmelectroluminescent element according to claim 4, wherein the divalentmetal element A is at least one selected from a group consisting of Pb,Sr and Ba.
 6. A thin film electroluminescent element according to claim5, wherein the divalent metal element is Pb.
 7. A thin filmelectroluminescent element comprising a phosphor thin film, a dielectricthin film disposed on at least one surface of said phosphor thin film,and electrodes for applying a voltage across said films, wherein thedielectric thin film comprises a first dielectric thin film layer whichis subject to dielectric breakdown of the self-healing type, expressedby the general formula of AB₂ O₆ where A represents a divalent metalelement selected from the group consisting of Pb, Sn, Mg, Ca, Sr, Ba, Znand Cd, and B represents a pentavalent metal element selected from thegroup consisting of Ta and Nb, and a second dielectric thin film layersuperimposed thereon, wherein said second dielectric thin film has aproduct E_(b) ·ε.sub.γ of dielectric breakdown electric field intensityE_(b) and dielectric constant ε.sub.γ not smaller than 80×10⁶ V/cm andis not susceptible to a dielectric breakdown of the self-healing type.8. A thin film electroluminescent element according to claim 7, whereinthe second dielectric thin film, not susceptible to the dielectricbreakdown of the self-healing type, is formed from a dielectric materialcontaining perovskite type titanate as a main component.
 9. A thin filmelectroluminescent element according to claim 4, wherein the divalentmetal element A is at least one selected from a group consisting of Pb,Sr and Ba.
 10. A thin film electroluminescent element according to claim9, wherein the divalent metal element is Pb.
 11. A thin filmelectroluminescent element according to claim 7, wherein the divalentmetal element A is at least one selected from a group consisting of Pb,Sr and Ba.